Heat-sealable laminar thermoplastic films

ABSTRACT

A biaxially oriented polypropylene film is provided which exhibits improved heat-seal strength and good optical clarity. The polypropylene film is characterized by having on at least one surface thereof a continuous coating of a heat-sealable film layer comprising a copolymer of ethylene and methylacrylate. The lamination may be formed by machine direction orientation of an uncoated polypropylene base web followed by extrusion coating with the copolymer material and subsequent transverse direction orientation of the combined layers.

REFERENCE TO COPENDING APPLICATION

This is a division of application Ser. No. 248,937, filed Mar. 30, 1981now U.S. Pat. No. 4,367,112, which is a continuation-in-part ofapplication Ser. No. 99,171, filed Nov. 30, 1979.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to laminated thermoplastic films. Inparticular, oriented polypropylene films are provided with a relativelythin surface coating of heat-sealable polymer material. The laminatedoriented polypropylene film exhibits good heat-seal strength, broadheat-seal temperature range, and excellent optical properties.

2. Description of the Prior Art

Biaxially oriented polypropylene flms have been widely used in the pastas packaging films, especially in the area of food packaging, since suchfilms offer excellent optical, mechanical, and barrier properties.Expanded usage of such films, however, has been limited by the narrowheat-seal range thereof. Such films, moreover, at the elevatedtemperature requisite for heat sealing, characteristically pucker andtear. A means used in the past to overcome this and to allow such filmsto be satisfactorily sealed over a wide temperature range has been toapply heat-sealable coatings to the surface of the film. For example,the polypropylene has been coextruded with surface layers of anethylene-propylene random copolymer and subsequently oriented. Althoughsuch films exhibit high seal strength, because of the relatively highmelting point of the copolymer skin, the temperature range over whichusable seals are formed is narrow. Similarly, oriented coextruded filmhas been prepared in which the skins are comprised of low densitypolyethylene which has been polymerized in a free radical-catalyzedprocess. In such case, because the melting point of the polyethyleneskin is lower than that of the ethylene-propylene copolymer, a broadersealing range can be achieved. However, such films have shown pooradhesion between the skin and core layers showing an increased tendencyto delaminate in heat-sealing operations. Consequently, the sealstrengths provided by skins comprised of polyethylene which has beenpolymerized by a free radical-catalyzed process are not as high as thoseprovided by an ethylene-propylene random copolymer skin. Moreover, thehaze and gloss values of such films are poorer than those exhibited byuncoated oriented polypropylene films.

Additional problems encountered when attempting to form a coextrudedbiaxially oriented polyethylene coated polypropylene film include thetendency of the lower melting point polyethylene to adhere to thesurface of draw rolls commonly employed to achieve machine directionorientation of the laminate. Attempts to remedy this problem aredisclosed in U.S. Pat. No. 4,147,827, the disclosure of which isincorporated herein by reference. In that patent, the core polypropylenematerial is blended with another resin which has a plasticizing effectupon the modified polypropylene. This allows the coextruded base web,carrying the polyethylene coating, to be stretched at temperatures belowwhich adhesion of the coating to the draw rollers occurs. Machinedirection stretch at such low temperatures, however, may, underparticular operating conditions, result in fibrillation of the modifiedpolypropylene core material whereby the resultant biaxially orientedlaminate may exhibit impaired physical properties. The following Table 1sets forth specific physical property improvements when an unmodifiedpolypropylene base web is compared to a polypropylene core layer whichhas been modified by the addition of a plasticizing agent, described bythe manufacturer as an interpolymer of alpha-methylstyrene; aliphatic C₅olefins; and terpenes.

                  TABLE 1                                                         ______________________________________                                        Property        Plain Core                                                                              Modified Core                                       ______________________________________                                        *WVTR           0.35      0.45                                                Ball Burst Strength                                                                           15        1                                                   (ASTM-905-37)                                                                 MO Ultimate (Psi)                                                                             23,000    2,000                                               Shrinkage at 275° F.                                                                   5 to 7%   10 to 20%                                           ______________________________________                                         *Water Vapor Transmission Ratio                                          

It can be seen from the foregoing Table that attempts to modify thepolypropylene base layer to effect machine direction orientation at lowtemperatures, result in a base layer having markedly reduced physicalstrength characteristics as well as lower barrier properties andundesirable shrinkage characteristics.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a biaxiallyoriented polypropylene film that is heat-sealable over a wide range oftemperatures, that provides a high level of heat-seal strength, and thathas good mechanical and optical properties. This is accomplished byforming a laminate consisting essentially of a biaxially orientedunmodified polypropylene core and at least one surface layer whichcomprises a low density, highly-branched copolymer of ethylene andmethylacrylate (hereafter "EMA"). This copolymer can be produced by thehigh pressure free radical copolymerization of ethylene andmethylacrylate.

A continuous method has been devised for manufacturing heat-sealablepolypropylene film comprising the steps of forming a continuous thinfilm substrate comprising a substantially uniform layer of polypropylenehomopolymer and uniaxially orienting the polypropylene layer by machinedirection stretching of the film substrate. A thin coating oflow-melting ethylene-copolymer is applied to the uniaxially orientedfilm substrate, the copolymer consisting essentially of about 6 to 30weight percent methylacrylate and about 94 to 70 percent ethylene. Thecoated film is biaxially oriented by stretching the film transversely tothe machine direction at a temperature above the melting point (about87° C. to 104° C.) of the ethylene copolymer.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In several of the following examples oriented film laminates wereprepared by forming the skin and core resins into individual plaques bycompression molding to the desired thickness. A composite, two-plystructure was then assembled, by joining the desired skin and corematerials by heat sealing around the periphery of the assembly. Thisheat sealing step was simply to ensure that the individual plies did notbecome separated during handling prior to orientation. Biaxialorientation was accomplished by stretching the laminar assembly on acommercially available T. M. Long orientation apparatus. The heat sealtests reported in the following Table were carried out by sealing thecoated surface of one laminate to the coated surface of anotherlaminate.

Additionally, laminates in several of the following examples were formedby coextrusion of the skin and core resins utilizing conventionalcoextrusion techniques followed by solidification of the coextrudate byquenching in a water bath or on a casting roll; reheating the thusformed base sheet; orienting the laminate in the machine directionutilizing conventional machine direction orientation rollers andsubsequently stretching the machine direction oriented assembly in thetransverse direction on a conventional tenter frame. Alternatively, asindicated in the following examples, samples were also preparedutilizing an extrusion coating process whereby a single layer ofpolypropylene was extruded, solidified, and oriented in the machinedirection. Subsequently, the skin resin was then melt-coated orextrusion coated on the already machine direction oriented basepolypropylene web. The composite film was subsequently oriented in thetransverse direction utilizing a tentering device.

The following examples are by way of illustration only and accordinglyshould not be construed in a limiting sense.

EXAMPLE 1

A 2.5-3 mil compression molded plaque of a copolymer of ethylene andmethyl acrylate (2.0 Melt Index, 20% methyl acrylate content) and a 35mil compression molded plaque of a polypropylene (0.905 density, 4.5MFR) were assembled as described above. The assembly was inserted in theT. M. Long stretching apparatus, and following preheating for one minuteat 315° F., was first stretched 5× at a rate of about 25,000%/min. (MD)and then was stretched 7× in a perpendicular direction to the firststretch at a rate of 3600%/min. (TD).

EXAMPLE 2

A coextrudate comprising a core of a polypropylene (0.905 density, 4.5Melt Flow Rate) and surface layers of a copolymer of ethylene and methylacrylate of Example 1, said surface layers constituting about 4% of thetotal sheet thickness per surface, was extruded and quenched on acasting roll at 40°-44° C. The quenched basesheet was then orientedbiaxially in the T. M. Long stretching apparatus the same way as inExample 1.

EXAMPLE 3

The procedure of Example 1 was followed except that the skin resin was acopolymer of ethylene and methyl acrylate (6 Melt Index, 18% methylacrylate content), having a melting point of 87° C.

EXAMPLE 4

The procedure of Example 1 was followed except that the skin resin was acopolymer of ethylene and methyl acrylate (2 Melt Index, 6% methylacrylate content), having a melting point of 104° C., that was producedby high pressure free radical polymerization.

EXAMPLE 5

The procedure of Example 1 was followed except that the skin resin was alow density polyethylene homopolymer produced by the freeradical-catalyzed process (2.4 Melt Index, 0.924 density).

EXAMPLE 6

A 1.5 mil laminar film was produced by extrusion-coating a copolymer ofethylene and methylacrylate (having a Melt Index of about 2.0 andcontaining about 20% by weight of methylacrylate) onto one side of apolypropylene base sheet (0.905 density, 4.5 Melt Flow Rate) which hasalready been oriented in the machine direction utilizing a draw ratio of5:1. The resultant composite film was subsequently stretched in thetransverse direction utilizing a transverse draw ratio of about 8:1 andstretching temperatures of between about 120° C. and 150° C. The layerof coating resin constituted approximately 20% by volume of the totallaminar film thickness.

EXAMPLE 7

The procedure of Example 6 was followed except that the coatingconstituted about 10% by volume of the total film thickness.

EXAMPLE 8

The procedure of Example 7 was followed except that the total filmthickness was 1.25 mils and the coating constituted about 15% by volumeof the total film thickness.

EXAMPLE 9

The procedure of Example 6 was followed except that the coatingconsisted of an ethylene-ethylacrylate (EEA) copolymer manufactured byUnion Cabide Corp., having a melt index of 8 and a melting point of 130°C. For this copolymer, a minimum seal temperature (ESM) of 270° F. to280° F. was required to obtain satisfactory seal strength, whereas, forthe EMA-coated polypropylene a minimum seal temperature of about 190° F.gives equivalent sealing properties.

The optical properties and sealing characteristics of biaxially orientedcoextruded films produced in accordance with the preceding examples areset forth in the following Table II.

                                      TABLE II                                    __________________________________________________________________________    OPTICS AND SEALABILITY OF                                                     POLYPROPYLENE - ETHYLENE/METHYL ACRYLATE COPOLYMER LAMINATES                                    SEAL STRENGTH, gm/in. - °F.                          EXAMPLE NO.                                                                            HAZE                                                                              GLOSS                                                                              180                                                                              190                                                                              200                                                                              210                                                                              220                                                                              230                                                                              240                                                                              250                                                                              260                                                                              270                                                                              280                                                                              290                        __________________________________________________________________________    1        1.6 91.0 65 125                                                                              225                                                                              325                                                                              360                                                                              425                                                                              470                                                                              555                                                                              635                                                                              710                                                                              825                                                                              670                        2        --  --   25 145                                                                              240                                                                              275                                                                              355                                                                              375                                                                              440                                                                              445                                                                              490                                                                              535                                                                              610                                                                              530                        3        2.4 88.5 0  40 135                                                                              185                                                                              285                                                                              315                                                                              390                                                                              450                                                                              475                                                                              550                                                                              595                                                                              555                        4        1.5 92.2 0  0  0  55 120                                                                              170                                                                              180                                                                              235                                                                              370                                                                              370                                                                              365                                                                              345                        5        2.3 90.7 0  0  0  0  0  0  65 222                                                                              360                                                                              413                                                                              410                                                                              355                        6        1.0 --   120                                                                              440                                                                              500                                                                              1340                                                                             1410                                                                             1630                                                                             1810                                                                             1730                                                                             1400                                                                             1370                                                                             1540                                                                             1920                       7        1.6 --   40 230                                                                              340                                                                              600                                                                              890                                                                              1120                                                                             1040                                                                             1110                                                                             1210                                                                             1440                                                                             1400                                                                             1410                       8        1.5 --   40 260                                                                              390                                                                              730                                                                              910                                                                              1100                                                                             1300                                                                             1200                                                                             1440                                                                             1440                                                                             1340                                                                             1400                       9        --  --   minimum seal temperature (ESM) = 270° F. to                            280° F.                                              __________________________________________________________________________

The preceding Table II illustrates that laminates prepared from acopolymer of ethylene and methylacrylate as a skin resin on apolypropylene substrate layer exhibit excellent heat sealability andoptical properties which are far superior to polypropylene laminateshaving a skin resin formed from a low density polyethylene homopolymer(see Example 5).

Examples 6, 7, and 8 illustrate that when an extrusion coating processis employed to fabricate the multilayer EMA-coated polypropylene of thepresent invention, exceptionally high heat seal characteristics areexhibited. Additionally, as shown in Example 6, although a 10% coatingthickness is sufficient to impart adequate heat seals, Examples 7 and 8show that seal strengths improve with an increase in coating thickness.

It has been found that the ethylene methylacrylate copolymer skin resinsthat are particularly suitable for employment in the laminar films ofthe present invention include those which exhibit a Melt Index fromabout 2 up to about 6 and a methylacrylate content by weight of fromabout 6% up to about 30%, preferably about 18 to 20%. These EMAcoolymers have a relatively low melting point. The preferred copolymers(e.g., 18% MA) melt at about 87° C. to 88° C., which provides a verybroad range of seal temperature to obtain minimum seal strength. Withlower amounts of MA comonomer (e.g., 6%), the melting point is higher,up to 104° C.

When the laminations of the present invention are employed in overwrappackaging applications, the total laminar film thicknesses are usuallyin the order of about 0.5 up to about 2.0 mils and preferably in therange of 0.7 up to about 1.5 mils. The total ethylene methylacrylatecopolymer skin thickness employed in such constructions may vary fromabout 0.01 mil up to about 0.7 mil with a preferred skin thickness rangeof from about 0.02 up to about 0.30 mil.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to, without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A method for heat sealing a film combination comprising applying to the surface of a biaxially oriented film of stereoregular polypropylene a coating consisting essentially of a copolymer of ethylene and methyl acrylate containing from about 6% up to about 30% by weight of methacrylate; placing the coated film into contact with another so-coated polypropylene film surface wherein the contacting surfaces are both coated, and heat sealing the combination at a temperature below about 270° F.
 2. A method for heat sealing a film combination comprising the steps of:forming a continuous thin film substrate comprising a substantially uniform layer of polypropylene homopolymer; uniaxially orienting the polypropylene layer by machine direction stretching of the film substrate; applying by melt coating or extrusion coating a thin coating of low-melting ethylene copolymer to the uniaxially oriented film substrate, said copolymer consisting essentially of about 6 to 30 weight percent methyl acrylate and about 94 to 70 percent ethylene, and having a minimum seal temperature between about 180° F. and below about 270° F.; biaxially orienting the coated film by stretching the film transversely to the machine direction at a temperature above the melting point of the ethylene copolymer; cooling the film to recover a biaxially oriented coated film; and placing the coated film into contact with another so-coated polypropylene film surface wherein the contacting surfaces are both coated, and heat sealing the combination at a temperature below about 270° F. 